1. Field of the Invention
The present invention relates to a method for removing a hydrocarbon solvent from a hydrocarbon solvent solution of a hydrogenated block copolymer. More particularly, the present invention is concerned with a method for removing a hydrocarbon solvent from a hydrocarbon solvent solution of a hydrogenated block copolymer which is obtained by hydrogenating a block copolymer comprising at least one polymer block composed mainly of vinyl aromatic monomer units and at least one polymer block composed mainly of conjugated diene monomer units, wherein the vinyl aromatic monomer units and the conjugated diene monomer units are contained in a specific amount ratio, wherein the hydrogenated block copolymer has a specific degree of hydrogenation, and wherein the method comprises the steps of: (1) mixing a hydrocarbon solvent solution of the hydrogenated block copolymer with heated water to obtain a dispersion of the hydrocarbon solvent solution in the heated water, and (2) introducing the dispersion into a steam stripping vessel containing heated water, wherein the dispersion is spouted into the gaseous phase in the steam stripping vessel toward the surface of the heated water in the steam stripping vessel under conditions wherein the vapor pressure P1 of the dispersion and the pressure P2 of the gaseous phase in the steam stripping vessel satisfy the formula: P1xe2x89xa7P2, thereby effecting a steam stripping of the dispersion to remove the hydrocarbon solvent from the dispersion and obtain porous crumbs of the hydrogenated block copolymer.
By the method of the present invention, desired porous crumbs can be stably and efficiently produced for a long period of time. Further, by subjecting the porous crumbs produced by the method of the present invention to dehydration and drying, dried porous crumbs can be continuously produced stably for a long period of time. The dried porous crumbs thus produced have not only excellent handling properties but also a low water content and a high oil-absorbing capability, so that the dried porous crumbs can absorb a satisfactory amount of a liquid additive, such as a softening agent or a silicone oil. Therefore, when the dried porous crumbs are used as a modifier in the production of a molding resin composition from a thermoplastic resin and a liquid additive, such as a softening agent or a silicone oil, a shaped article having an excellent appearance can be produced by molding the molding resin composition.
2. Prior Art
A hydrogenation product of a block copolymer comprising a polymer block composed mainly of vinyl aromatic monomer units and a polymer block composed mainly of conjugated diene monomer units (hereinafter, the hydrogenation product is frequently referred to simply as a xe2x80x9chydrogenated block copolymerxe2x80x9d) not only has excellent properties with respect to weathering resistance, heat resistance and impact resistance, but also exhibits, even if not vulcanized, excellent strength and excellent elasticity both at room temperature, which are comparable to those of a conventional vulcanized rubber. Therefore, the hydrogenated block copolymer is widely used in various application fields, such as daily sundry commodities, automobile parts, parts for light electric appliances, and various products for industrial use.
Hydrogenated block copolymers are commercially available in various forms, such as pellets and crumbs. In general, crumbs of a hydrogenated block copolymer are used as a modifier in the production of a molding resin composition from a thermoplastic resin and a liquid additive, such as a softening agent or a silicone oil. For obtaining a shaped article having an excellent appearance from a molding resin composition containing a modifier, it is necessary that the crumbs used as the modifier have the capability of rapidly absorbing a satisfactory amount of a liquid additive, such as a softening agent or a silicone oil.
In general, the polymerization and hydrogenation for producing a hydrogenated block copolymer are performed in a hydrocarbon solvent inert to a polymerization catalyst and a hydrogenation catalyst, so that the hydrogenated block copolymer produced is obtained in the form of a uniform solution or suspension thereof in the hydrocarbon solvent. Therefore, it is necessary to remove the solvent from the solution or suspension. With respect to the method for removing the solvent, various methods are known. As an example of such methods, there can be mentioned a method called a xe2x80x9csteam stripping methodxe2x80x9d, which comprises adding to hot water a solution or suspension of a hydrogenated block copolymer in a solvent, and distilling off from the resultant mixture the solvent together with steam. By removing the solvent by this method, crumbs of the hydrogenated block copolymer can be obtained.
Unexamined Japanese Patent Application Laid-Open Specification No. 50-89494 (corresponding to U.S. Pat. No. 4,087,484) describes a method for removing a solvent from a hydrocarbon solvent solution of a hydrogenated block copolymer, comprising effecting a steam stripping in the presence of a specific surfactant. Unexamined Japanese Patent Application Laid-Open Specification No. 61-218614 describes a method for producing crumbs of a hydrogenated block copolymer, comprising removing a solvent from a hydrocarbon solvent solution of a hydrogenated block copolymer by steam stripping, and removing volatile components from the resultant by means of a twin-screw vented extruder. Unexamined Japanese Patent Application Laid-Open Specification No. 8-301929 (corresponding to U.S. Pat. No. 5,616,652) describes a method for producing porous crumbs of a hydrogenated block copolymer, comprising subjecting a heated hydrocarbon solvent solution of a hydrogenated block copolymer to steam stripping.
However, these methods are defective in that a polymer which has become viscous due to the removal of the solvent is likely to adhere to the steam stripping vessel or to be agglomerated. Therefore, by any of these methods, it is impossible to continuously produce crumbs stably for a long period of time.
Examined Japanese Patent Application Publication No. 2-18321 discloses a method comprising mixing a polymer solution with heated water in the presence of a surfactant and/or a dispersant to obtain an emulsion, and spouting the emulsion into the gaseous phase in a steam stripping vessel. However, when this method is applied to the removal of a solvent from a solution of a hydrogenated block copolymer, the following disadvantages arise. The wet crumbs of the hydrogenated block copolymer which are produced by this method have too small a particle diameter. Therefore, problems occur not only in that the recovery of the crumbs is difficult, but also in that, when the wet crumbs are subjected to dehydration, the dehydration of the crumbs becomes unsatisfactory. Hence, the operation cannot be efficiently conducted. Thus, it is difficult to continuously produce dried crumbs stably for a long period of time by using the wet crumbs produced by this method.
As described hereinabove, by any of the conventional methods for removing a solvent, it is impossible to continuously produce porous crumbs stably for a long period of time.
In this situation, the present inventors have made extensive and intensive studies with a view toward solving the problems accompanying the prior art. As a result, it has unexpectedly been found that stable and efficient production of desired porous crumbs can be continuously performed for a long period of time by removing a hydrocarbon solvent from a hydrocarbon solvent solution of a hydrogenated block copolymer in accordance with the undermentioned method. The hydrogenated block copolymer is obtained by hydrogenating a block copolymer comprising at least one polymer block composed mainly of vinyl aromatic monomer units and at least one polymer block composed mainly of conjugated diene monomer units, wherein the vinyl aromatic monomer units and the conjugated diene monomer units are contained in a specific amount ratio, wherein the hydrogenated block copolymer has a specific degree of hydrogenation. The method comprises the steps of: (1) mixing a hydrocarbon solvent solution of the hydrogenated block copolymer with heated water to obtain a dispersion of the hydrocarbon solvent solution in the heated water, and (2) introducing the dispersion into a steam stripping vessel containing heated water, wherein the dispersion is spouted into the gaseous phase in the steam stripping vessel toward the surface of the heated water in the steam stripping vessel under conditions wherein the vapor pressure P1 of the dispersion and the pressure P2 of the gaseous phase in the steam stripping vessel satisfy the formula: P1xe2x89xa7P2, thereby effecting a steam stripping of the dispersion to remove the hydrocarbon solvent from the dispersion and obtain porous crumbs of the hydrogenated block copolymer. It has also been found that, by subjecting the porous crumbs produced by this method to dehydration and drying, dried porous crumbs having not only excellent handling properties but also a low water content and a high oil-absorbing capability can be continuously produced stably for a long period of time. The dried porous crumbs thus produced can absorb a satisfactory amount of a liquid additive, such as a softening agent or a silicone oil. Therefore, when the dried porous crumbs are used as a modifier in the production of a molding resin composition from a thermoplastic resin and a liquid additive, such as a softening agent or a silicone oil, a shaped article having an excellent appearance can be produced by molding the molding the resin composition. Based on these findings, the present invention has been completed.
Accordingly, it is a primary object of the present invention to provide a method for continuously producing porous crumbs stably and efficiently for a long period of time which porous crumbs are advantageous in that, when the porous crumbs are subjected to dehydration and drying to produce dried porous crumbs, the dried porous crumbs exhibit desired properties, such as excellent handling properties, a low water content and a high oil-absorbing capability.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawing.